In H2S syntheses from hydrogen and sulfur, polysulfanes (H2Sn) are generally found as by-products in the crude gas in an order of magnitude of ≧400 vpm (volume per million) and when the gas stream is compressed these tend to decompose in an uncontrolled manner into H2S and sulfur. This leads to undesirable sulfur deposits in the whole of the compression area, including the peripheral pipes and valves.
It is known that polysulfanes are thermodynamically unstable and have a tendency to decompose, particularly when heated (M. Schmidt, W. Siebert: “Sulfane” in Comprehensive Inorganic Chemistry, vol. 2, sect. 2.1, Pergamon Press, Oxford 1973, 826-842). Traces of alkali on the surface of glass vessels lead to the deposition of elemental sulfur. However, these findings come from investigations with polysulfanes present in more or less pure form. In principle, of course, they also apply to polysulfanes present in great dilution. In this case, however, the influence of the concentration ratios has to be taken into account.
In the crude gas compositions mentioned above, the polysulfanes are present in great dilution in hydrogen sulfide, which at the same time represents a decomposition product of the polysulfanes in a thermodynamic equilibrium:
      x    ⁢                  ⁢          H      2        ⁢          S      n        ⇌            x      ⁢                          ⁢              H        2            ⁢      S        +                            x          ⁡                      (                          n              -              1                        )                          8            ⁢              S        8            
The high H2S concentration makes the shift of the equilibrium to the left-hand side seem probable and the decomposition of the polysulfanes into hydrogen sulfide and sulfur to be a non-preferred reaction.
The object of the invention is to provide a process for the practically complete removal of the polysulfanes, thus preventing sulfur deposits in the plant's pipes.